He would always laugh, he said, because people always expected him to have the answer, always hoped he simply could say why things had gone the way they had.

After all, he started at least a season's worth of games at all five positions on the offensive line during his NFL career. He is believed to be the only player in NFL history to do that. And he played in 296 games in all, in 42 NFL stadiums, spanning 19 seasons, with 14 Pro Bowl selections.

And Hall of Fame offensive lineman Bruce Matthews never once missed a game because of an injury.

Asked for his analysis, Matthews once said: "I was just blessed with a body that could take a pounding. That's all I can say. I'm not sure I had much to do with that, but I'm glad it happened that way."

Offensive line is a high-impact position, filled with large, powerful players with unnerving quickness for their size, working in close quarters. There is a reason it has been referred to, down through the decades, as the trenches.

It is where the dirty work gets done. And to play in there for almost two decades without being forced to miss even a single regular-season weekend with an injury, may be one of the most miraculous feats in league history.

"I'm starting to understand the difference between playing the offensive line in college and in the NFL," Broncos rookie center J.D. Walton said. "Everybody is big, everybody is strong and everybody is fast for their size."

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Most of the guys up front play with knee braces, to try to prevent injury or because they've already had one. The knees always are the hot zone, carrying 300-pound players around, driving into other 300-pounders as well as moving side to side and up the field.

Ankles get twisted, and the shoulders and wrists take a beating too. Some interior linemen, especially centers, have neck trouble as the years go by.

Most linemen prefer run blocking because they are driving out, delivering the blows, dealing out the punishment. In pass-blocking sets, they often are in a position to "catch" the charging defensive linemen.

As the years go by, the aches and pains often compound. Former Broncos guard Mark Schlereth's 29 surgeries (20 on his knees) are an extreme testimony to the difficulty of playing the position.

"It's a physical, demanding job," Broncos right tackle Ryan Harris said. "Those are big guys you're going against, and they are trying to push you out of the way."

Get your peak on

Physical peak: 28

Many NFL executives put the age here but pointed out that most of the tackles in the 2010 Pro Bowl, including the Broncos' Ryan Clady, were younger. It is, of course, based on those who stay injury-free in their first four or five seasons, meaning they have already beaten the odds to have stayed in the league that long.

Mental peak: 32

Many players on the offensive line are disappointed that just when their grasp of the game and their ability to make split-second decisions in traffic is at its highest because of all of their experience, their bodies are also breaking down.

Prime age: 30

Again, they have to make it this far with their knees, ankles and shoulders intact. NFL centers usually play longer than linemen at other positions, so the prime time of their careers can come a little later.

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